6-O-methyl erythromycin A (clarithromycin) is a semisynthetic macrolide antibiotic related to erythromycin A. Clarithromycin exhibits excellent antibacterial activity against gram-positive bacteria, some gram-negative bacteria, anaerobic bacteria, Mycoplasma, and Chlamydia. It is stable under acidic conditions and is efficacious when administered orally. Clarithromycin is useful therapy for infections of the upper respiratory tract in children and adults. Clarithromycin is stable under acidic conditions and is efficacious when administered orally.
The chemical structure of clarithromycin is: 
Several crystal forms of clarithromycin and/or solvates of clarithromycin, “Form 0,” “Form I”, and “Form II” have been identified as indicated in U.S. Pat. No. 5,945,405. The crystal form of clarithromycin is typically identified by the powder x-ray diffraction patterns. Different crystalline forms of clarithromycin may have different thermal stability, cost of preparation, dissolution characteristics and bioavailability.
Various methods of preparing clarithromycin described in the patent literature have been reported to result in different forms of clarithromycin. Purification of crude clarithromycin has been reported to convert one form of clarithromycin to another form. For example, methods in which the compound is purified by recrystallization from ethanol, have been reported to result in the initial formation of Form 0 solvate (see U.S. Pat. No. 5,945,405) or in the initial formation of Form I (See U.S. Pat. No. 5,858,986). The Form 0 solvate may be used as a therapeutic agent, as described in U.S. Pat. No. 5,945,405. The '405 patent discloses that the Form 0 solvate may be converted to the non-solvated Form I by removing the solvent by drying at a temperature of from about 0° C. to about 50° C. Another form, Form II, is reported to be relatively thermodynamically stable compared to Form I. The clarithromycin currently marketed in the United States under the trademark Biaxin® is formulated using Form II.
Several methods of converting clarithromycin Form 0 or Form I to Form II have been described. One such method, as described in U.S. Pat. Nos. 4,945,405 and 5,858,986, is to heat Form 0 under vacuum at a temperature of between about 70° C. and 110° C. According to this patent, the Form 0 solvate first converts to Form I clarithromycin and then to Form II. This method is described in U.S. Pat. Nos. 4,945,405 and 5,858,986. However, this vacuum drying step of converting Form 0 to Form II is expensive in both energy and material handling. Clarithromycin Form II has been reported to be formed when Form I is crystallized from various solvents, including ethanol and water (U.S. Pat. No. 5,844,105).